Variable inductance devices



Jan. 20, 1959 A. BICKERS v 2,870,423

VARIABLE mnucmncrs DEVICES Filed March 28, 1955 all] Mu. 32m

United States Patent 2,870,423 VARIABLE INDUCTANCE DEVICES Arthur Bickers, Chelmsferd, England, assignor to Marconis Wireless Telegraph Company Limited, London,

England, a British company Application March 28, 1955, Serial No. 497,294 Claims priority, appiication Great Britain April 21, 1954 2 Claims. (Cl. 336-110) This invention relates to variable inductance devices.

Variable inductance devices are frequently required for various purposes in the radio and allied arts, for example as variable tuning elements in tuned circuits. Where there are a number of tuned circuits required to be tuned to the same variable frequency or to difiierent variable frequencies in pre-determined fixed frequency difference relationship, it is a usual requirement that the tuning elements be ganged i. e. controlled from the same handle. It is also generally required that the range of variation of inductance of an inductance device shall be large but that, nevertheless, the device shall be small and compact. It is very difficult to satisfy these requirements without involving undue cost and complexity and the common form of variable inductance device employed in radio engineering, namely, that consisting of a coil with a movable ferro-magnetic core, falls far short of practical requirements in many cases.

According to this invention there is provided a variable inductance device comprising a permanent magnet, a ferromagnetic circuit member for providing a path for the lines of force of said magnet, at least one coil arranged substantially wholly within an air gap or gaps in said member, and means for moving a ferro-magnetic element included in the magnetic circuit so as to vary the flux density in the circuit member, and thereby the inductance of the coil or coils.

The movable ferro-magnetic member may be the permanent magnet or a fixed permanent magnet and a separate movable ferro-magnetic member may be used.

Preferably the movable ferro-magnetic member is rotatably mounted in an additional air gap in the ferromagnetic circuit member.

Preferably the coil, or coils, has or have a core or cores made of a ferro-magnetic material.

Preferably a magnetic screen or shield is provided to confine the leakage field which is produced most strongly when the movable ferro-magnetic member is in positions of low inductance adjustment.

A preferred form of ganged inductance device unit in accordance with the invention comprises a plurality of coils differently positioned along the path for the lines of force and a single movable ferromagnetic member for varying the flux density in said path and thereby the inductance values of all the coils simultaneously.

For a better understanding of the invention, and to show how the same may be carried into effect reference will now be made to the accompanying drawing, in which:

Fig. 1 is a sectional view of a variable inductance device embodying the invention;

Fig. 2, a horizontal sectional view taken on the line II-II of Fig. 1; and

Fig. 3, a vertical sectional view on the line III-III of Fig. 1 with the actual windings omitted.

Referring now to Fig. 1 of the drawing, a permanent magnet 1 is rotatably mounted between the ends 2 and 3 of a main yoke 4, upon a spindle 5, there being small air gaps 6 and 7 between the magnet 1 and the yoke ends 2 and 3 respectively. One end of the spindle is mounted by a pivot bearing 8 in a brass block 9, which latter flanks one side of the ends 2 and 3 of the yoke 4. A second brass block 10 likewise flanks the other side of the 2,870,423 Patented Jan. 20, 1959 ends 2 and 3. The block 10 has an aperture therethrough serving as a journal for the spindle 5. The spindle 5 is urged against the pivot bearing 8 by means of a spring washer 11 interposed between the face 11 of the magnet 1 and the inside lateral face of the brass block 10.

A leakage field screen 12, the plane of which is substantially at right angles to the axis of the magnet 1,' is symmetrically positioned about the magnet 1. One side 12a of the screen 12 abuts against the brass block 9, while the other side 12b abuts against the block 10, the latter side having an aperture through which the spindle 5 passes. The screen 12 provides a magnetic circuit for leakage field from the magnet 1, and thus reduces the liability for leakage field to link with one or more coils 13 located in the magnetic circuit of the main yoke 4. The cores of the coils 13 may have the same initial core permeability and the same number of turns, and may be retained in position by a suitable potting resin.

Preferably the cores of the coils are cylindrical tubes, they may conveniently be of the material known as ferroxcube, and dimensioned to fit fairly closely between the pole pieces formed in the yoke 4 to receive them. The coils 13 are wound round their cores after the manner of a flattened toroid as indicated, the coil cores being indicated at 13a and the said pole pieces at 1312. The internal dimensions of the yoke 12 are such that the magnet 1 may rotate therein with clearance. A brass strap 14 flanks each side of the yoke 4 to hold the parts of said yoke lying between the coils 13.

It will be seen that the magnetic circuit of the device is completed by way of the magnet 1, air gap 6, the yoke end 2, the coils 13, the yoke end 3, the air gap 7 and the magnet 1. Thus by rotating the magnet 1, the magnetic flux threading the circuit will be altered, and

hence the inductance of the coils will be varied together.-

While I have described my invention in certain preferred embodiments, I realize that modifications may be made, and I desire that it be understood that no limitations upon my invention are intended other than may be imposed by the scope of the appended claims.

I claim:

1. A variable inductance device comprising a ferromagnetic circuit, said circuit including a permanent magnet arranged as a series element in said circuit and constituting a movable member rotatable therein within an air-gap, a plurality of coils arranged substantially wholly within a plurality of air-gaps in said circuit, and a magnetic screen located adjacent said movable member whereby the leakage field from the permanent magnet is substantially prevented from affecting the inductance of said coils.

2. A variable inductance device as set forth in claim 1 wherein said ferro-magnetic circuit member is U-shaped, the end of each arm of the U having an inwardly directed part, wherein the permanent magnet constituting the movable member is rotatably mounted between said parts, there being a clearance between the magnet and the parts such as to provide said additional air-gap, the arrangement being such that when the poles of said magnet are in line with the parts, substantially all the flux of the magnet threads said member, and wherein the plurality of coils are positioned in said member, the arrangement being such that rotation of said movable member varies the inductance of all the coils simultaneously.

References Cited in the file of this patent UNITED STATES PATENTS 1,896,510 Given Feb. 7, 1933 2,000,378 Deisch May 7, 1935 2,380,242 Jewell July 10, 1945 2,581,202 Post Jan. 1, 1952 

